X-Ray absorption spectroscopy of H₃O

Area of Science:

• Physical Chemistry
• Atomic and Molecular Physics
• Spectroscopy

Background:

• Understanding the electronic structure of water clusters and ions is crucial for various chemical and physical processes.
• X-ray absorption spectroscopy (XAS) is a powerful tool for probing electronic states, but its application to isolated molecular ions requires specialized techniques.

Purpose of the Study:

• To investigate the O 1s X-ray absorption spectra of isolated hydronium (H3O+) cations.
• To compare the spectral features of H3O+ and H2O+ with neutral H2O.
• To provide experimental data that can aid in the identification of molecular species in aqueous systems.

Main Methods:

• Preparation of isolated H3O+ and H2O+ cations using a flowing afterglow ion source.
• Measurement of O 1s X-ray absorption spectra for these ions.
• Theoretical calculations using time-dependent density functional theory (TD-DFT) for spectral analysis.

Main Results:

• Significant differences observed in resonance energies and widths between ionic and neutral water spectra.
• Resonances for H3O+ shifted to higher energies by up to 10 eV compared to neutral H2O.
• Resonance widths increased by up to a factor of 5 for the studied cations.
• TD-DFT calculations showed good agreement with experimental spectral data.

Conclusions:

• The distinct spectral signatures of H3O+ and H2O+ can be used for their identification.
• This work provides a foundation for using XAS to study the composition of charged water clusters and liquid water.
• The findings advance the understanding of electronic structure in hydrated ions.